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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.beans;

/**
 * An <code>Expression</code> object represents a primitive expression
 * in which a single method is applied to a target and a set of
 * arguments to return a result - as in <code>"a.getFoo()"</code>.
 * <p>
 * In addition to the properties of the super class, the
 * <code>Expression</code> object provides a <em>value</em> which
 * is the object returned when this expression is evaluated.
 * The return value is typically not provided by the caller and
 * is instead computed by dynamically finding the method and invoking
 * it when the first call to <code>getValue</code> is made.
 *
 * @author Philip Milne
 * @see #getValue
 * @see #setValue
 * @since 1.4
 */
public class Expression extends Statement {

  private static Object unbound = new Object();

  private Object value = unbound;

  /**
   * Creates a new {@link Expression} object
   * for the specified target object to invoke the method
   * specified by the name and by the array of arguments.
   * <p>
   * The {@code target} and the {@code methodName} values should not be {@code null}.
   * Otherwise an attempt to execute this {@code Expression}
   * will result in a {@code NullPointerException}.
   * If the {@code arguments} value is {@code null},
   * an empty array is used as the value of the {@code arguments} property.
   *
   * @param target the target object of this expression
   * @param methodName the name of the method to invoke on the specified target
   * @param arguments the array of arguments to invoke the specified method
   * @see #getValue
   */
  @ConstructorProperties({"target", "methodName", "arguments"})
  public Expression(Object target, String methodName, Object[] arguments) {
    super(target, methodName, arguments);
  }

  /**
   * Creates a new {@link Expression} object with the specified value
   * for the specified target object to invoke the  method
   * specified by the name and by the array of arguments.
   * The {@code value} value is used as the value of the {@code value} property,
   * so the {@link #getValue} method will return it
   * without executing this {@code Expression}.
   * <p>
   * The {@code target} and the {@code methodName} values should not be {@code null}.
   * Otherwise an attempt to execute this {@code Expression}
   * will result in a {@code NullPointerException}.
   * If the {@code arguments} value is {@code null},
   * an empty array is used as the value of the {@code arguments} property.
   *
   * @param value the value of this expression
   * @param target the target object of this expression
   * @param methodName the name of the method to invoke on the specified target
   * @param arguments the array of arguments to invoke the specified method
   * @see #setValue
   */
  public Expression(Object value, Object target, String methodName, Object[] arguments) {
    this(target, methodName, arguments);
    setValue(value);
  }

  /**
   * {@inheritDoc}
   * <p>
   * If the invoked method completes normally,
   * the value it returns is copied in the {@code value} property.
   * Note that the {@code value} property is set to {@code null},
   * if the return type of the underlying method is {@code void}.
   *
   * @throws NullPointerException if the value of the {@code target} or {@code methodName} property
   * is {@code null}
   * @throws NoSuchMethodException if a matching method is not found
   * @throws SecurityException if a security manager exists and it denies the method invocation
   * @throws Exception that is thrown by the invoked method
   * @see java.lang.reflect.Method
   * @since 1.7
   */
  @Override
  public void execute() throws Exception {
    setValue(invoke());
  }

  /**
   * If the value property of this instance is not already set,
   * this method dynamically finds the method with the specified
   * methodName on this target with these arguments and calls it.
   * The result of the method invocation is first copied
   * into the value property of this expression and then returned
   * as the result of <code>getValue</code>. If the value property
   * was already set, either by a call to <code>setValue</code>
   * or a previous call to <code>getValue</code> then the value
   * property is returned without either looking up or calling the method.
   * <p>
   * The value property of an <code>Expression</code> is set to
   * a unique private (non-<code>null</code>) value by default and
   * this value is used as an internal indication that the method
   * has not yet been called. A return value of <code>null</code>
   * replaces this default value in the same way that any other value
   * would, ensuring that expressions are never evaluated more than once.
   * <p>
   * See the <code>execute</code> method for details on how
   * methods are chosen using the dynamic types of the target
   * and arguments.
   *
   * @return The result of applying this method to these arguments.
   * @throws Exception if the method with the specified methodName throws an exception
   * @see Statement#execute
   * @see #setValue
   */
  public Object getValue() throws Exception {
    if (value == unbound) {
      setValue(invoke());
    }
    return value;
  }

  /**
   * Sets the value of this expression to <code>value</code>.
   * This value will be returned by the getValue method
   * without calling the method associated with this
   * expression.
   *
   * @param value The value of this expression.
   * @see #getValue
   */
  public void setValue(Object value) {
    this.value = value;
  }

  /*pp*/ String instanceName(Object instance) {
    return instance == unbound ? "<unbound>" : super.instanceName(instance);
  }

  /**
   * Prints the value of this expression using a Java-style syntax.
   */
  public String toString() {
    return instanceName(value) + "=" + super.toString();
  }
}
